﻿606 MK. J. V. ELSDEN ON THE IGNEOUS ROCKS [Aug. I905, 



2. The intrusive rocks are of later date, and are of three distinct 

 types, namely: 



(a) The gabbros and diabases of the Strurable-Head area. 



(b) The norites and associated rocks of St. David's Head and the 



surrounding district. 



(c) The lime-bostonites and porphyrites of the Abercastle-Mathry district. 



S. The lime-bostonite series is apparently older than the gabbros 

 and norites, and seems to belong to the petrographical 

 province of the South-East of Ireland. 



4. The gabbros and norites were intruded approximately during 



the same interval, at a later period. ' 



5. The norites and associated rocks have spread north-eastward 



from St. David's Head, and have penetrated the area of the 

 Strumble-Head intrusions. 



6. The gabbros and diabases have similarly spread to a limited 



extent south-westward into the norite-area. 



7. The gabbro- and norite-provinces are separated by an ill-defined 



zone, in which some mixture of the magmas took place. 



8. The latest phase of igneous activity was the formation of the 



Pen-Caer basaltic laccolite, with apophyses penetrating the 

 Garn-Fawr Y-Garn intrusions. 



It is not, of course, necessary to assume that each of the several 

 intrusions was confined to any one single stage of vulcanicity. 

 The laccolites and bosses were probably the result of injections, 

 extending over a prolonged interval, from co-existing magma-basins 

 or from a single differentiated magma. There are clear evidences 

 of some further differentiation in situ., but the full extent to which 

 this took place offers a large field for future investigation. 



In conclusion, I have to express my indebtedness to Mr. G. T. 

 Holloway for placing his laboratory at my disposal, and thus greatly 

 facilitating the chemical work connected with the preparation of 

 this paper. 



EXPLANATION OF PLATES XXXVIII-XL. 



Plate XXXYIII. 



Geological sketch-map of the country between Fishguard and St. David's Head, 

 on the scale of 1| miles to the inch. 



Plate XXXIX. 



-Fig. 1. Biotite-norite from the western end of Carn Llidi, showing ' herring- 

 bone' twin of augite. The minerals represented are plagioclase, 

 augite, rhombic pyroxene, and ilmenite. X 30 diams. (See p. 585.) 



2. The same rock, showing large plates of rhombic pyroxene enclosing 



plagioclase and intergrown with augite. X 30 diams. (See p. 585.) 



3. Quartz-norite, Penberry Hill, with conspicuous bronzite. The felspars 



are cloudy, and the augite is granular. X 30 diams. (See p. 588.) 



4. Albite-enstatite rock, Porth-Gain Quarry. The enstatite is much altered, 



and is interstitial to the felspar. X 30 diams. (See p. 592.) 



5. Enstatite-diorite, with micropegmatite, Garnedd Lleithr. X 30 diams. 



(Seep. 587.) 



